1. Field of the Invention
The present invention relates to synchronization equipment, and more particularly to synchronization equipment which, after simple hardware installation and software setting, establishes a synchronization network with an existing network and SDH (Synchronous Digital Hierarchy) elements, etc. that do not support SSMB (Synchronization Status Message Half Byte).
2. Description of the Related Art
In recent years, SDH elements that use an SSMB signal to switch from one timing source to another for network synchronization have been increasing in number. The SSMB signal is transmitted using the low-order four bits of the S1 byte (formerly Z1 #1 byte) carried in the multiplex section overhead (MSOH) of an STM-n (Synchronous Transfer Mode-n) signal.
As shown in FIG. 7, described subsequently, the SSMB information itself is defined in the binary code table of ITU-T G.708, where four-bit signals, in various combinations of four bits, are each defined in relation to SDH synchronization quality. For example, "0010 (02h)" indicates the synchronization quality equivalent to G.811 (primary timing source using a cesium atomic oscillator), and "1111" is defined as "Don't use for sync."
FIGS. 1A to 1C show an operational example of a network that supports SSMB.
In FIG. 1A, network element 1 (NE1) sets priority 1 for an external clock input (External Input) and priority 2 for an input line 2 (Line 2). Further, network element 2 (NE2) sets priority 1 for an input line 1 (Line 1) and priority 2 for an input line 4 (Line 4). Similarly, network element 3 (NE3) sets priority 1 for an input line 3 (Line 3) and priority 2 for an external clock input (External input). In the initial state, the network elements 1, 2, and 3 are each set to select the priority 1 side.
The network element 1 is connected to a primary synchronization clock generator (Primary Clock) constructed from a cesium atomic oscillator, and sends the SSMB value "02h", representing the synchronization quality based on the primary clock, to its downstream network element 2 as the S1 byte signal in the STM multiplex section overhead on Line 1. Likewise, the network element 2 sends the SSMB value "02h" to the network element 3 as the S1 byte signal in the STM multiplex section overhead on Line 3. Here, the SSMB value of Lines 2 and 4 is set to "0Fh" to prevent a timing loop. In this way, the network elements 1, 2, and 3 are slave-synchronized to the primary synchronization clock signal from the primary synchronization clock generator.
Next, if a failure occurs in the primary synchronization clock generator or on its output line, as shown in FIG. 1B, the network element 1 is put in a holdover state and remains in that state. With this change in the synchronization quality, the network element 1 changes the SSMB value from "02h" to "0Bh" (SETS--Synchronization Equipment Timing Source), and sends the SSMB value to its downstream network element 2 via Line 1. The network element 2 sends the same SSMB value "0Bh" to its downstream network element 3 via Line 3. As a result, the entire network is synchronized to the holdover of the network element 1.
In FIG. 1C, the network element 3 compares the synchronization quality (SSMB value "0Bh) on Line 3 of priority 1 with the synchronization quality (SSMB value "04h") of the clock being applied at the external clock input (External Input) of priority 2 from a secondary synchronization clock generator (Secondary Clock) constructed from a rubidium atomic oscillator, selects the priority 2 side providing the better equality, and sends the SSMB value "04h" to the network element 2 via Line 4. The network element 2 performs a similar comparison to select the better quality priority 2 side, and sends the SSMB value "04h" to the network element 1 via Line 2.
Next, the network element 1 selects the priority 2 side providing the higher synchronization quality than its own holdover. As a result, the network elements 1, 2, and 3 are now slave-synchronized to the secondary synchronization clock signal from the secondary synchronization clock generator. Further, the SSMB value of Lines 1 and 2 is changed to "0Fh" to prevent a timing loop.
FIGS. 2A and 2B show an example of an environment where a network and SDH elements that support SSMB coexist with an existing network and SDH elements that do not support SSMB.
As shown in FIGS. 2A and 2B, in the existing SSMB non-supporting network and SDH elements (indicated by oblique hatching), "1111" indicating a not-used state is often set in the S1 byte. Accordingly, in the SSMB supporting network and SDH elements (indicated by dots) that are slaved to them, a decision is made that the timing source cannot be used ("Don't use for sync."). As a result, there has been the problem that synchronization cannot be established successfully for the SSMB supporting network and SDH elements slaved to the SSMB non-supporting network and SDH elements.